1. Field of the Invention
The invention is directed to an ATM switching equipment having a switching network, an input interface unit containing an input processing unit, at least one output interface unit containing an output processing unit and having a microprocessor, in which the switching equipment is configured to write a new VPI/VCI information for the further connecting section into the cells of the arriving data streams upon utilization of routing tables.
2. Description of the Related Art
Asynchronous transfer mode (ATM) is a network technology that is suited for the transport of all known signal data such as pure data, voice and video data, etc.
ATM may be utilized as the connection-oriented packet switching method for B-ISDN (broadband integrated services digital network). Structuring equal-length cells is characteristic for ATM. Information to be communicated is divided onto ATM cells that comprise 53 bytes, which includes: a cell header with 5 bytes and payload information of 48 bytes. The header information identifies a specific virtual connection and all cells follow the transmission path determined as a result this virtual connection identification that is defined at the set up of the connection by virtual connections in a network.
An overview of ATM is provided, for example, by “ATM-Networks, Concepts, Protocols and Applications” by Händel, Huber and Schröder, Addison-Wesley-Longman, 2nd Edition, 1994 (ISBN 0-201-42274-3).
In contrast to a TDMA method in which time slots are assigned in advance to different types of data traffic, incoming data traffic at an ATM interface is segmented into said 53-byte cells, and these cells are sequentially forwarded in the way they were generated. The routing of the cells through a network occurs upon utilization of the routing information stored in the cell header (see FIG. 1)
FIG. 1 shows the exemplary structure of the field of an ATM cell. In the case of a user network interface UNI, the first 4 bits contain the “generic flow control” GFC, and contain a virtual path identifier VPI in the case of a network node interface NNI. Another 4 bits VPI follow in this structure in the first line, 4 more bits in the next line, and a total of 2 bytes for a virtual channel identifier VCI.
In the fourth byte (or the fourth line), the VCI information are followed by another three bits of information with respect to the payload type PT and a one-bit information with respect to the cell loss priority CLP; and the last byte of the header contains the header error control HEC. The header is followed by said 48 bytes of payload INF.
The VCI information serves for distinguishing between the various logical channels in a switching section; the VPI information relates to channel bundles that are respectively composed of a plurality of virtual channels, by which cells within a bundle can be rapidly processed by a switching network in an exchange. The PT information identifies the type of payload field, by which the information field of the cell must also be interpreted in a switching center in addition to the header field; but payload data contained therein are skipped in an exchange.
The header error control HEC is nothing more than a checksum that identifies errors in the header of the cell that, for example, are produced by transmission errors. The information CLP references ATM cells that are of less significance and can be potentially discarded for buffer overflows.
Routing tables are employed when switching ATM cells, by which the controller of an exchange: 1) acquires the information VCI and VPI from the arriving cell, 2) respectively determines the information VPI, VCI for the following connecting section with the assistance of the routing tables, and 3) enters them in the cell header, and the cell is forwarded to the output of the exchange.
A layer model has also been specified for the ATM technology by which a distinction is made between the physical layer, the actual ATM layer, the ATM adaption layer AAL and higher layers (higher layer protocols). This layer model is presented, for example, in “ATM, Solutions for Enterprise Internetworking”, David Ginsburg, Addison-Wesley 1996, ISBN 0-201-87701-5, in which the corresponding recommendations of ITU are also referenced in Chapter 2.2.
In addition to these four layers, the AAL layer being of particular interest in the framework of the invention, a distinction is also made between three different levels, namely the user level, the control level, and the administration level.
The AAL layer adapts the higher-ranking layers to the ATM layer and implements: 1) the subdivision of the data streams into cells at the transmission side, and 2) the merging to form messages at the reception side. Different running times are also compensated by the AAL layer.
For uniformity, the protocols of the AAL layer have been divided into various classes and, with respect to this, Chapter 3.1.4 “Adaption” in the above work by Ginsburg or “Mobilftinknetze und Ihre Protokolle”, Volume 2, B. Walke, Täubner Stuttgart 1998, ISBN 3-519-06431-6, Chapter 8.2.5, “ATM-Dienstklassen”, can be referenced. However, in addition to the four service groups originally specified by ITU, there is also an AAL5 group that was initially optimized for the data transport on the part of the industry and whose development is ongoing in the ITU standardization process (see Ginsburg, p. 83).
Class 2 is especially of interest in the framework of the invention; this class deals with the AAL2 protocol used, for example, for real-time services with variable and low bit rate. The inherently two-layer structure that has both the ATM layer as well as the AAL2 layer is unique for AAL2. Accordingly, A two-layer switching, namely an ATM switching and an AAL2 switching, must occur in a switching node.